1. Field of the Invention
This invention relates to the detection of temperature within a semiconductor, and, more particularly, to a structure having a sensor located in close proximity to the source and/or drain electrodes of an MOS transistor or to the emitter and/or collector electrodes of a bipolar transistor.
2. Description of Related Art
Advances in the art of semiconductor processing have increased the density of components on a chip and the power that they deliver. Although package designs attempt to dissipate heat away from the chip, there are conditions where it is necessary to know the temperature of critical elements on a semiconductor chip to ensure performance within design parameters or to prevent physical damage to the semiconductor structure itself.
This is particularly important when a fault condition such as an output short circuit condition is encountered. The problem is aggravated in high voltage dielectrically isolated circuits where the heat generating devices are located in individual tubs formed by a few microns of silicon dioxide which are under and around a device. Silicon dioxide is an excellent dielectric material, but its thermal resistivity is about 100 times that of silicon, so it interferes with dissipating heat to a heat sink on the back of the chip.
Attempts have been made to integrate thermal sensors on the chip. These are typically a diode or bipolar transistor located next to the heat generating device. The thermal sensor interacts with a shutdown circuit which makes a comparison to a reference voltage and shuts down the power if a limit is exceeded.
The problem with the structure above is that there is a temperature gradient along the chip from this heat generating device to the sensor. This gradient is precipitously increased if a dielectric isolation tub is encountered on the way to the sensor. Therefore, the sensor detects a temperature which may be significantly lower than that of the actual device. Similarly, there is a temporal lag between the onset of a fault condition and the detection of increased temperature by the sensor located nearby the device.
Accordingly, there is a need in the art for a semiconductor structure which reacts faster to thermal overload conditions and which more accurately detects the maximum temperature generated by the device.